The present invention relates to a method of reclaiming crosslinked rubber and to a molding of reclaimed rubber.
Heretofore, there has been known a reclaiming method wherein wastes of rubber moldings such as tire wastes etc., or scrap, defective moldings, etc. occurring in a process for producing rubber moldings, are converted into reclaimed rubber by applying heat and shear stress thereto.
According to the above reclaiming method, crosslinkages among rubber molecules constituting crosslinked rubber are cleaved by heat and shear stress, thus providing reclaimed rubber in a state similar to non-crosslinked rubber.
By re-crosslinking such reclaimed rubber singly and/or re-crosslinking a mixture which is formed by mixing new non-crosslinked rubber and the reclaimed rubber, moldings of the reclaimed rubber is formed, thus enabling recycling of crosslinked rubber.
A certain kind of crosslinked rubber generates decomposed products during reclamation, and a deterioration in qualities etc. may occur owing to the decomposed products. In the conventional reclaiming method, it was difficult to deal with the problem caused by the decomposed products.
Specifically, a certain kind of crosslinked rubber generates a malodorous gas during reclamation. To reclaim such crosslinked rubber, (1) the malodorous gas is subjected to combustion and deodorization in a combustion-type deodorizing unit and is released to the atmosphere as shown in JP-A 6-210633, and (2) malodorous components in the malodorous gas and in reclaimed rubber are continuously removed by heating in a heating oven as shown in JP-A 9-308951.
In the method (1), the malodorous gas could be removed from the gas released to the atmosphere during reclamation, but the malodorous gas remaining in the reclaimed rubber could not be removed. Accordingly, there was a problem that a bad smell is generated upon re-crosslinking of the reclaimed rubber or genrated from moldings of the re-crosslinked reclaimed rubber.
Removal of the malodorous gas by the method (2) is also unsatisfactory, and it was difficult in some cases to obtain reclaimed rubber applicable to articles used particularly in a closed space (e.g. in a room). Further, use of a heating oven made it difficult to apply the method to crosslinked rubber easily denatured by heating.
In a certain kind of crosslinked rubber in an embodiment different from the above, its decomposed products produced during reclamation react again with the crosslinked rubber in the process of reclamation to cause a deterioration in the qualities of reclaimed rubber. Further, the decomposed products can be a cause for premature crosslinkage to deteriorate scorch characteristics and storage stability.
The present invention was made in view of such conventional problems, and the object of the present invention is to provide a method of reclaiming crosslinked rubber wherein a reduction of qualities due to generation of decomposed products hardly occurs, and a molding of reclaimed rubber.
The present invention relates to a method of reclaiming crosslinked rubber, which includes introducing a degasification carrier and removing, together with the degasification carrier, decomposed products in crosslinked rubber, during a reclamation step of reclaiming crosslinked rubber by applying shear stress thereto and/or a subsequent step after the reclamation step.
The most remarkable feature of the present invention lies in removal of decomposed products together with the introduced degasification carrier.
According to this feature, the decomposed products are removed, thus scarcely remaining in crosslinked rubber in the process of reclamation and in reclaimed rubber. Accordingly, the deterioration in qualities of reclaimed rubber by the decomposed products and the inhibition of reclamation of crosslinked rubber by the decomposed products can be prevented.
According to the present invention, as described above, there can be provided a method of reclaiming crosslinked rubber wherein a reduction in qualities due to generation of decomposed products hardly occurs.
The process for introducing the degasification carrier and for removing decomposed products together with the degasification carrier is carried out preferably during reclamation of crosslinked rubber (see Embodiment 6). With this arrangement, the effect of the present invention can be secured.
As described later, cleavage of crosslinkages in crosslinked rubber is initiated by applying shear stress thereto, and almost simultaneously with the cleavage of crosslinkages, the decomposed products are also generated. Therefore, the effect of the present invention cannot be obtained if introduction and removal of the degasification carrier (degasification treatment) is conducted before decomposed products are generated, that is, before the cleavage of crosslinkages in crosslinked rubber is initiated. Accordingly, the degasification treatment should be carried out after a plasticizing step and/or a kneading step, which are described later. The degasification treatment may be repeated.
The decomposed products include e.g. malodorous components.
A certain kind of crosslinked rubber generates a malodorous gas during reclamation. As described above, it was difficult to remove the malodorous gas from such crosslinked rubber by the conventional method. The reclaiming method of the present invention can be applied to remove the malodorous components thereby improving the working environment during reclamation or during re-crosslinking of reclaimed rubber in forming moldings of reclaimed rubber (by rolling refining etc.)
Further, when the reclaimed rubber is formed into moldings of reclaimed rubber, the bad smell of the moldings of reclaimed rubber can be reduced, and thus the value of the moldings can be increased.
Further, the ratio of reclaimed rubber in moldings of the reclaimed rubber can be increased to improve the efficiency of recycling.
In addition, the processability of reclaimed rubber can be raised to improve crosslinking characteristics. Furthermore, the physical characteristics of reclaimed rubber moldings obtained from the reclaimed rubber can also be improved. Moreover, reclamation itself can also be promoted.
The crosslinked rubber generating malodorous gas as described above includes sulfur-vulcanized EPDM (ethylene propylene diene terpolymer), NR/SBR (blended rubber of natural rubber and styrene-butadiene rubber), SBR (styrene-butadiene rubber), NBR (acrylonitrile-butadiene rubber) etc.
When the crosslinked rubber generating a malodorous gas as described above is reclaimed, a deodorant may be added thereto. The malodorous gas can thereby be further reduced. The deodorant in this case can make use of perfumes such as vanillin and lignin and deodorants such as activated carbon and sepiolite.
Besides, rubbers such as acryl rubber generating amine compounds as decomposed products can be reclaimed by the production method of the present invention.
The amount of the degasification carrier added is preferably 0.02 to 20 parts by weight based on 100 parts by weight of crosslinked rubber. Decomposed products can thereby certainly removed. If the amount of the degasification carrier is less than 0.02 parts by weight, the effect is hardly obtained, while if the amount exceeds 20 parts by weight, the amount of the degasification carrier volatilized can be increased to make stable reclamation of crosslinked rubber difficult and to make removal of the degasification carrier incomplete, thus permitting a large amount of the degasification carrier to remain in the reclaimed rubber. The lower limit of the degasification carrier is preferably 0.1 part by weight, more preferably 0.5 part by weight. The upper limit thereof is preferably 7 parts by weight.
In the step of removing decomposed products together with the degasification carrier, techniques selected from heating degasification, degasification under reduced pressure, solvent cleaning, and bubbling are preferably used alone or in combination thereof.
The heating degasification is a method of removing decomposed products by gasification; the degasification under reduced pressure is a method of removing decomposed products by promoting gasification thereof under reduced pressure; the solvent cleaning is a method of removing decomposed products by dissolution; and the bubbling is a method of promoting removal of decomposed products by increasing the gasification area of decomposed products.
The degasification under reduced pressure is conducted preferably by using a low-boiling compound as the degasification carrier in order to remove decomposed products together with the degasification carrier. With this arrangement, gasification of the low-boiling compound and decomposed products, particularly malodorous component, can be promoted under reduced pressure to realize efficient removal of the decomposed products.
Particularly when the decomposed products are malodorous components, it is preferable that by treatment with the degasification carrier, the decomposed products are reduced to xc2xd or less relative to those before introduction of the degasification carrier. The decomposed products are reduced more preferably to ⅓ or less, most preferably {fraction (1/10)} or less.
With this arrangement, it is possible to improve the working environment under which the reclaimed rubber is formed into e.g. moldings of reclaimed rubber. Further, when the reclaimed rubber is formed into the moldings of reclaimed rubber described below, the bad smell of the moldings of reclaimed rubber can be reduced, whereby the value of the moldings can be increased.
Particularly when the malodorous components are sulfur compounds or nitrogen compounds, it is preferable that they are reduced by degasification to ⅓ or less relative to those before introduction of the degasification carrier. They are reduced more preferably to {fraction (1/20)} or less by degasification.
The reclamation step of reclaiming the crosslinked rubber by applying shear stress thereto and the step of removing the decomposed products with the degasification carrier are conducted preferably successively. With this arrangement, efficient reclamation of the crosslinked rubber can be realized.
Now, the reclamation step of reclaiming the crosslinked rubber by applying shear stress thereto is described.
This reclamation step consists of a preheating step, a plasticizing step and a kneading step. The preheating step is a step in which the crosslinked rubber is heated to a temperature at which the crosslinkages therein can be cleaved. The plasticizing step is a step in which cleavage of the crosslinkages in the crosslinked rubber is initiated thereby initiating softening of the crosslinked rubber. The kneading step is a step in which the crosslinkages are cleaved and the rubber molecules are dispersed and mixed.
The shear stress in the plasticizing step is preferably 1 to 100 MPa. With this arrangement, reclamation of crosslinked rubber can be certainly effected. When the shear stress is less than 1 MPa, the shear stress is too small to effect promotion of cleavage of crosslinkages and thus the efficiency of reclamation may be lowered. On the other hand, when the shear stress is larger than 100 MPa, the shear stress permits progress of not only cleavage of crosslinkages but also cleavage of main chains of rubber molecules, and thus the physical properties of reclaimed rubber may be deteriorated. The upper limit of the shear stress in the plasticizing step is more preferably 15 MPa.
The reclamation step is conducted preferably at the temperature of 100 to 520xc2x0 C. With this arrangement, the crosslinkages are cleaved while cleavage of the main chains is suppressed.
In the plasticizing step, the temperature of the crosslinked rubber is preferably 100 to 520xc2x0 C. With this arrangement, reclamation of the crosslinked rubber can be certainly effected. When the temperature of the crosslinked rubber is lower than 100xc2x0 C., cleavage of the crosslinkages may not sufficiently proceed. On the other hand, when the temperature is higher than 520xc2x0 C., cleavage of the main chains proceeds, and thus the physical properties of the reclaimed rubber may be deteriorated. The upper limit of the temperature of the crosslinked rubber is more preferably 450xc2x0 C.
In the reclamation step, the crosslinked rubber is heated or cooled as necessary so as to be in the above temperature range. When the heat evolved by the crosslinked rubber upon shearing is too low, the rubber is heated; and when the heat is too high, the rubber is cooled. If the above temperature range is attained by the heat of the crosslinked rubber upon shearing, heat transfer (heating or cooling) from the outside is not necessary.
The optimum temperature range is varied depending on the type of crosslinked rubber. For example, when automobile tires etc. are to be reclaimed, the temperature range is preferably 80 to 360xc2x0 C. When peroxide-crosslinked rubbers such as EPDM are to be reclaimed, the temperature range is preferably 220 to 450xc2x0 C.
The upper limit of the temperature range during reclamation is varied depending on the duration of reclamation, and the temperature should be raised for shorter reclamation.
The step of reclaiming the crosslinked rubber is conducted preferably by use of an extruder. With this arrangement, reclamation of the crosslinked rubber can be successively effected to realize efficient reclamation.
By use of an extruder, degasification can be performed in the extruder, and the frequency of contact between the degasification carrier and the crosslinked rubber or reclaimed rubber can be raised, thus enabling efficient reclamation and degasification.
When the crosslinked rubber is resin-crosslinked butyl rubber, the present invention is preferably used.
When the resin-crosslinked butyl rubber is reclaimed by the conventional method, decomposed products consisting of phenol resin etc. occur and react again with resin-crosslinked butyl rubber in the process of reclamation, and thus the resultant reclaimed rubber is poor in qualities.
The deterioration in qualities of the reclaimed rubber, caused by re-reaction of the decomposed products with resin-crosslinked butyl rubber in the process of reclamation, can be prevented by the reclaiming method of the present invention. Further, a deterioration in the scorch characteristics and storage stabilities of the reclaimed rubber can also be prevented.
The degasification carrier is preferably at least one selected from a group including inert gas, water and alcohol. The effect of the present invention can thereby be secured.
Particularly, water can be used to achieve the effect of promoting decomposition of crosslinked rubber by water and of hydrolyzing the decomposed products into harmless ones. Further, the cleaning effect of water, the increase of the gasification area due to the bubbling effect of gasification, and the effect of water vapor as a carrier gas can be obtained, and thus the decomposed products can be efficiently removed. Further, water is inexpensive, thus reducing costs.
The inert gas includes N2, Ar, He, CO2 etc.
The present invention also relates to a molding of reclaimed rubber produced by a process including introducing a degasification carrier and removing, together with the degasification carrier, decomposed products in crosslinked rubber to obtain reclaimed rubber during a reclamation step of reclaiming crosslinked rubber by applying shear stress thereto and/or a subsequent step after the reclamation step, and re-crosslinking the reclaimed rubber or melt-blending the reclaimed rubber with thermoplastic resin.
By removing the decomposed products together with the introduced degasification carrier, the decomposed products are removed thus scarcely remaining in crosslinked rubber in the process of reclamation and in reclaimed rubber. Accordingly, the deterioration in qualities of reclaimed rubber by the decomposed products is prevented, and as a consequence, the deterioration in qualities of moldings of reclaimed rubber produced by re-crosslinking the reclaimed rubber or by melt-blending the reclaimed rubber with thermoplastic resin can be prevented.
Other detailed features are similar to those described above.
According to the present invention, there can be provided moldings of reclaimed rubber wherein deterioration of qualities due to generation of decomposed products hardly occurs, as described above.
When the crosslinked rubber is resin-crosslinked butyl rubber, the present invention is preferably used.
In reclamation of resin-crosslinked butyl rubber, the deterioration in qualities of the resultant reclaimed rubber, caused by re-reaction of decomposed products such as phenol resin with resin-crosslinked butyl rubber in the process of reclamation, can be prevented according to the present invention. Further, a deterioration in the scorch characteristics and storage stability of the reclaimed rubber can also be prevented. Accordingly, moldings of reclaimed rubber excellent in storage stability and superior in molding-workability can be obtained.
The amount of decomposed products in the reclaimed rubber is reduced preferably to xc2xd or less relative to that before introduction of the degasification carrier. The deterioration in qualities by the decomposed products can there by be prevented.